The present invention relates to a semiconductor device including microparticles and the like for holding charge to permit use of the semiconductor device as a memory.
Existing ultralarge-scale integrated circuits (ULSI) have a memory section where a number of memory devices each composed of a MOS transistor are integrated. In recent years, demands have increased on the memory devices for higher operation speed, reduced power consumption, and prolonged recording retention. To respond to these demands, development of MOS transistors satisfying these demands is in progress.
Memory devices proposed and trial-produced so far employ a technique that quite a small amount of charge is injected into a microparticle of a semiconductor or the like during write or erase operation of the memory and the charge is held therein. An example of this conventional technique is a study by S. Tiwari et al. on a memory using a plurality of silicon microparticles (dots) (Appl. Phys. Lett. 68 (1996) 1377).
FIG. 57 is a cross-sectional view of the conventional semiconductor memory device functioning as a memory using a plurality of silicon microparticles. The semiconductor memory device includes a tunnel oxide film 6202 made of SiO2 and a SiO2 film 6204 deposited in this order on a p-type silicon substrate 6201. An n-type polysilicon electrode 6205 is formed on the SiO2 film 6204. Silicon microparticles 6203 are buried between the tunnel oxide film 6202 and the SiO2 film 6204. Source/drain regions 6206 are formed in portions of the p-type silicon substrate 6201 located below both sides of the n-type polysilicon electrode 6205.
In the above conventional semiconductor memory device, by applying a positive voltage to the n-type polysilicon electrode 6205, electrons can be injected into the silicon microparticles 6203 via the tunnel oxide film 6202. By applying a negative voltage to the n-type polysilicon electrode 6205, electrons in the silicon microparticles 6203 can be released. This enables the threshold voltage of the device to change depending on whether or not electrons exist in the silicon microparticles 6203. The levels (high or low) of the threshold voltage are made associated with information H (high) and information L (low), respectively. In this way, information writing/reading is realized.
The thickness of the tunnel oxide film 6202 is extremely small (about 1.5 nm to 4 nm). Therefore, the above electron injection process is by direct tunneling, not Fowler-Nordheim (FN) tunneling.
According to a study by the inventors of the present invention, a highly sophisticated and fine fabrication technique is required to attempt to implement the above proposed semiconductor device with practical performance.
For example, if the tunnel oxide film 6202 is too thick, charge injection by tunneling is difficult, resulting in difficulty in obtaining low-voltage operation and high-speed operation. On the contrary, if the tunnel oxide film 6202 is too thin, charge confinement is insufficient during charge holding, resulting in difficulty in long-term charge holding, that is, long-term information recording.
In addition, in order to provide practical characteristics for the conventional semiconductor device, required is a fabrication technique allowing for high-level control of the diameter of the silicon microparticles 6203 as well as the dispersion thereof. If the diameter of the silicon microparticles 6203 is too small or large to provide a sufficient surface density of the silicon microparticles 6203, the charge holding duration is too short or the allowable amount of charge held is too small, resulting in reduction of the reliability of the semiconductor device.
Moreover, in the case of an increase of thermal energy due to temperature rise, for example, charge accumulated in the silicon microparticles 6203 is released spontaneously due to tunneling from the silicon microparticles 6203 to the p-type silicon substrate 6201.
In view of the above, in order to provide practical device characteristics for the above conventional semiconductor device, it is necessary to control the quality and thickness of the tunnel oxide film 6202 uniformly with markedly high precision. It is also necessary to provide the silicon microparticles 6203 at a high surface density in a uniform dispersion state while ensuring a uniform diameter of the silicon microparticles 6203. A highly sophisticated fabrication technique is required to realize the above control over the entire surface of the p-type silicon substrate. In consideration of the above, if the conventional semiconductor device is actually fabricated in the above fabrication process, the possibility that the resultant semiconductor device has practical characteristics is small. Further, the fabricated conventional semiconductor device will be poor in reliability. In short, according to the study by the inventors of the present invention, it is difficult for the conventional semiconductor device to realize high-speed charge injection and release and long-term charge holding.
An object of the present invention is providing a semiconductor device with high reliability that can be easily fabricated, and a method for fabricating such a semiconductor device.
The first semiconductor device according to the present invention include: a substrate having a conductive layer; and charge holding region including a barrier layer formed on the conductive layer for functioning as a barrier against charge transfer and a plurality of particles dispersed in the barrier layer so that the particles have different distances from the conductive layer from each other. The capacitance of the particles is larger as the distance from the conductive layer is smaller.
As the capacitance is larger, potential rise during charge holding is smaller and thus charge transfer is easier. Therefore, it becomes easy for the particles located farther from the conductive layer to hold charge, or to release charge upon application of a voltage. This charge holding state can be used as information.
In the first semiconductor device, the capacitance is larger and charge transfer is easier as the particles are dispersed at a higher dispersion density or as the mean diameter of the particles is larger. Therefore, by varying these factors depending on the distance from the conductive layer, spontaneous release of accumulated charge can be effectively suppressed, resulting in prolonged charge holding in the charge holding region. This increases the reliability.
In the first semiconductor device, the particles are quantized. This facilitates control of charge injection and release with a voltage.
Preferably, the plurality of particles are divided into a plurality of particle groups each composed of a plurality of particles common in the distance from the conductive layer.
The semiconductor device further includes: an insulating layer formed on the barrier layer; a gate electrode formed on the insulating layer; and source/drain regions formed by introducing an impurity into regions of the substrate located below both sides of the gate electrode. The resultant semiconductor device functions as a MIS transistor.
The second semiconductor device according to the present invention includes: a substrate having a conductive layer; and a charge holding region including a barrier layer formed on the conductive layer for functioning as a barrier against charge transfer and a plurality of particles dispersed in the barrier layer so that the particles have different distances from the conductive layer from each other. The barrier height of inter-particle portions of the charge holding region is smaller as the distance from the conductive layer is smaller.
As the barrier height is smaller, charge transfer is easier. Therefore, it becomes easy for the particles located farther from the conductive layer to hold charge, or to release charge upon application of a voltage. This charge holding state can be used as information.
The particles may have a smaller electron affinity or a larger sum of the electron affinity and the forbidden bandwidth as the distance from the conductive layer is smaller. This makes it possible to easily provide a difference in barrier height among the inter-particle portions.
Alternatively, the barrier layer may have a larger electron affinity or a smaller sum of the electron affinity and the forbidden bandwidth as the distance from the conductive layer is smaller. This also makes it possible to easily provide a difference in barrier height among the inter-particle portions.
Preferably, the plurality of particles are divided into a plurality of particle groups each composed of a plurality of particles common in the distance from the conductive layer.
The third semiconductor device according to the present invention includes: a substrate having a conductive layer; a first tunnel barrier film formed on the conductive layer; a quantized semiconductor layer formed on the first tunnel barrier film; a second tunnel barrier film formed on the first tunnel barrier film and the semiconductor layer; quantized semiconductor particles formed on the second tunnel barrier film in the state of being isolated from one another; and an insulating film formed on the second tunnel barrier film and the semiconductor particles.
The formation of the quantized semiconductor layer permits control of electron transfer between the quantized semiconductor particles and the conductive layer. That is, since charge transfer occurs only when the potential at the energy level of the semiconductor particles and the potential at the energy level of the semiconductor layer match each other, spontaneous release of charge accumulated in the semiconductor particles can be effectively suppressed, allowing for prolonged charge holding in the semiconductor particles. In addition, under presence of an appropriate electric field, charge can be easily injected from the conductive layer into the semiconductor particles and released from the semiconductor particles toward the conductive layer. In this way, control of charge injection into, holding in, and release from the semiconductor particles is ensured. The resultant semiconductor device is a highly reliable device that responds to the request for prolonged recording retention while satisfying the request for high-speed operation and reduced power consumption.
The discrete energy width of the semiconductor layer is preferably smaller than the discrete energy width of the semiconductor particles.
The semiconductor layer may be a particle group composed of particles in contact with one another.
The fourth semiconductor device according to the present invention includes; a substrate having a conductive layer; a first barrier layer formed on the conductive layer for functioning as a barrier for charge transfer; quantized first particles formed on the first barrier layer; a second barrier layer formed on the first particles for functioning as a barrier for charge transfer; and second particles formed on the second barrier layer. The diameter of the second particles is larger than the diameter of the first particles.
During charge holding, the particles having a large diameter have a higher charge holding ability and thus a lower charge release probability than the particles having a small diameter. Utilizing this characteristic, the second particles can be used as an information holder.
The ratio of the diameter of the second particles to the first particles is not less than 1.8 and not more than 300. This makes it possible to appropriately adjust the charge holding function of the second particles and the function of the first particles as a charge passing member.
The diameter of the first particles is preferably not less than 0.1 nm and not more than 5 nm. The diameter of the second particles is preferably not less than 1 nm and not more than 30 nm.
The fifth semiconductor device according to the present invention includes: a substrate having a conductive layer; a first barrier layer formed on the conductive layer for functioning as a barrier for charge transfer; first particles formed on the first barrier layer; a second barrier layer formed on the first particles for functioning as a barrier for charge transfer; and second particles formed on the second barrier layer. The potential at the second particles against charge to be held or released is lower than the potential at the first particles.
The second particles having a lower potential have a charge holding ability higher than the first particles. Utilizing this characteristic, the second particles can be used as an information holder.
When the charge to be held or released is electrons, the electron affinity of the second particles is preferably larger than the electron affinity of the first particles. Further, when the conductive layer is a semiconductor layer, the electron affinity of the second particles is preferably larger than the electron affinity of the semiconductor layer
When the charge to be held or released is holes, the sum of the electron affinity and the forbidden bandwidth of the second particles is preferably smaller than the sum of the electron affinity and the forbidden bandwidth of the first particles. Further, when the conductive layer is a semiconductor layer, the sum of the electron affinity and the forbidden bandwidth of the second particles is preferably smaller than the sum of the electron affinity and the forbidden bandwidth of the semiconductor layer.
The sixth semiconductor device according to the present invention includes: a substrate having a conductive layer; a first SiO2 layer formed on the conductive layer; a SiOxNy layer (0xe2x89xa6x less than 2, 0 less than yxe2x89xa64/3) formed on the first SiO2 layer; a second SiO2 layer formed on the SiOxNy layer; and particles formed on the second SiO2 layer.
Levels allowing for charge passing are generated in the vicinity of the interfaces between the SiOxNy layer and the first and second SiO2 layers and inside the SiOxNy layer. This enables the particles to hold charge and thus be used as an information holder.
The seventh semiconductor device according to the present invention includes: a substrate having a conductive layer; a first insulating layer formed on the conductive layer; first particles formed on the first insulating layer; a first barrier layer formed on the first particles for functioning as a barrier for charge transfer; second particles formed on the first barrier layer; a second barrier layer formed on the second particles for functioning as a barrier for charge transfer; and third particles formed on the second barrier layer. The diameter of each of the first and third particles is larger than the diameter of the second particles.
The charge holding function of the first and third particles is higher than the charge holding function of the second particles. Utilizing this, charge exchange is possible between the first particles and the third particles via the second particles as a charge passing member. Thus, the first and third particles can be used as an information holder.
The ratio of the diameter of each of the first and third particles to the second particles is not less than 1.8 and not more than 300. This makes it possible to appropriately adjust the charge holding function of the first and third particles and the charge passing function of the second particles.
The diameter of the second particles is preferably not less than 0.1 nm and not more than 5 nm. The diameter of each of the first and third particles is preferably not less than 1 nm and not more than 30 nm.
The eighth semiconductor device according to the present invention includes: a substrate having a conductive layer; a first insulating layer formed on the conductive layer; first particles formed on the first insulating layer; a first barrier layer formed on the first particles for functioning as a barrier for charge transfer; second particles formed on the first barrier layer; a second barrier layer formed on the second particles for functioning as a barrier for charge transfer; and third particles formed on the second barrier layer. The potential at each of the first and third particles against charge to be held or released is lower than the potential at the second particles.
The charge holding function of the first and third particles is higher than the charge holding function of the second particles. Utilizing this, charge exchange is possible between the first particles and the third particles via the second particles as a charge passing member. Thus, the first and third particles can be used as an information holder.
When the charge to be held or released is electrons, the electron affinity of each of the first and third particles is preferably larger than the electron affinity of the second particles. When the charge to be held or released is holes, the sum of the electron affinity and the forbidden bandwidth of each of the first and third particles is preferably smaller than the sum of the electron affinity and the forbidden bandwidth of the second particles.
The ninth semiconductor device according to the present invention includes: a substrate having a conductive layer; an insulating layer formed on the conductive layer; first particles formed on the insulating layer; a first SiO2 layer formed on the first particles; a SiOxNy layer (0xe2x89xa6x less than 2, 0 less than yxe2x89xa64/3) formed on the first SiO2 layer; a second SiO2 layer formed on the SiOxNy layer; and second particles formed on the second SiO2 layer.
Levels allowing for passing charge are generated in the vicinity of the interfaces between the SiOxNy layer and the first and second SiO2 layers and inside the SiOxNy layer. This enables the particles to hold charge and thus be used as an information holder.
The tenth semiconductor device according to the present invention includes: a substrate having a conductive layer; a barrier layer formed on the conductive layer for functioning as a barrier for charge transfer; a charge holder formed on the barrier layer; and quantized particles buried in at least part of the barrier layer.
It is possible to control charge transfer while using the particles as a charge passing member between the charge holder and the conductive layer. If quantized particles are used, in particular, the resultant semiconductor device is usable as a memory permitting high-speed writing and erasing.
The eleventh semiconductor device according to the present invention includes: a substrate having a conductive layer; a first SiO2 layer formed on the conductive layer; a SiOxNy layer (0xe2x89xa6x less than 2, 0 less than yxe2x89xa64/3) formed on the first SiO2 layer; a second SiO2 layer formed on the SiOxNy layer; and a charge holder formed on the second SiO2 layer.
Levels allowing for passing charge are generated in the vicinity of the interfaces between the SiOxNy layer and the first and second SiO2 layers and inside the SiOxNy layer. Therefore, the resultant semiconductor device is usable as a memory permitting high-speed write/erase operations.